Ultrasonography is a well-known medical diagnostic technique in which pulses of ultrasonic energy are directed into the body, and returning echoes are detected. In the past, medical systems using ultrasonography have produced black and white (gray scale) images illustrating the tissue surrounding portions of the vascular system. Tissue, being a relatively good reflector of ultrasound, appeared on images in varying shapes and shades of gray depending on the return echo amplitudes. Blood, however, being a poor reflector of ultrasound, would not appear in these images. The vessel interior, instead, appears black in the image where ultrasound was poorly reflected. A gray scale image alone does not provide sufficient information to diagnose the vascular system because soft plaque, which builds up in vessel walls and which is a poor reflector of ultrasound, cannot always be distinguished from the blood.
In the mid-1970's, ultrasound technology began to utilize the Doppler principle to obtain information on blood flow from moving scatterers in the blood. In one technique a continuous wave Doppler ultrasound signal is transmitted into the body and the echoed signal is processed to separate out the signals which are Doppler shifted (those having a change in frequency from the transmitted ultrasound signal). This continuous wave technique did not provide spatial information and was somewhat difficult to use. To improve spatial resolution, a pulsed Doppler ultrasound technique was developed, wherein reflected energy from a sample volume within a vessel is analyzed for changes in intensity and frequency relative to the transmitted ultrasound signal. Using the continuous wave or the pulsed Doppler technique, an analysis of one axial line or sample site is respectively performed. The output normally is a waveform showing the changes in frequency versus time and changes in intensity versus time. Neither pulsed nor continuous wave Doppler, however, provide real-time anatomical information about the vessel. A skilled technician analyzes the peak velocities of the intensity and the waveform spreading over time in the frequency versus time waveform for indications of disease.
One of the problems with these techniques is that the peak frequencies which are proportional to blood flow velocities may be inaccurate. When plaque builds up in the vessels, the direction and speed of blood flow are altered. The peak velocity at the sample site, as calculated parallel to the vessel, may have a direction different from the line of the vessel. For the case where plaque is not symmetrical, the velocity component parallel to the wall is an inaccurate measure when measured using the assumption that flow is of peak velocity.
Another problem with these methods is that a relatively few sample sites, usually one, are analyzed for blood flow characteristics.
Another problem is that the relative position of the sample site within the vessel as shown in a gray scale image may vary from the actual site within the vessel due to the movement of the blood vessel due to heart pulsation or the ultrasound transducer between samplings. Thus the gray scale image needs to be displayed in close to real time. This has imposed a limit on the number of sample sites that can be processed using conventional technology.
X-ray angiography, another vascular system diagnosis methodology, is a well-known technique in which a contrast dye (opaque to X-ray) is injected into the vascular system and X-ray pictures are taken of the dye passing through the vasculature. This approach is invasive and does not provide software tissue anatomical or blood flow velocity information needed for accurate disease diagnosis.
In summary, the contemporary evaluations of the vascular system are hampered by an inability to see all the influences on blood flow in a global fashion. X-ray angiography and tissue imaging provide a view of the vascular tree, but miss the details of flow within a vascular compartment and show none of the surrounding soft tissue. Continuous wave Doppler ultrasound provides evidence of flow events, but tells nothing about the non-anatomical characteristics of the vessel and surrounding tissue. Duplex imaging offers a view of the surrounding anatomy and waveform output of flow within a sample volume. A single sample volume and a corresponding waveform output provide little information about the dynamic flow within the vessel and the influences of those dynamics on the Doppler spectral output.